Positioning control system utilizing optical beam

ABSTRACT

A positioning control system for performing recording/reproducing operations by irradiating an optical beam to predetermined position on an optically recording medium, e,g., magneto-optical disk, includes a photo-detector constituted by at least two-divisional units; and a servo-error signal generating circuit which can generate at least one servo-error signal in accordance with a difference between the detection currents. Preferably, servo-error signal generating circuit includes at least one division circuit that has two pairs of transistors, two emitters in each pair of transistors being connected together into a common emitter, bias voltages of direct current type being applied to the respective bases of transistors on one side, the respective collectors of the transistors on one side being connected to a common connecting portion via resistors, the common connecting portion being connected to a power supply, the respective bases of transistors on the other side being connected together into a common base; and an integrating capacitor which is connected to the common base of transistors on the other side, and which integrates a difference between the current flowing through the common connecting portion and the reference current, to apply the thus integrated difference to the common base of transistors on the other side. Further, preferably, the photo-detector is connected to the common emitters in the respective pairs of transistors, so that the detection currents can flow through the common emitters.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a positioning control system, whichincludes a tracking servo control and a focusing servo control utilizingan optical beam in a magneto-optical disk device, an optical diskdevice, an optical card, or the like, and which enables a desired trackposition, etc., to be accurately determined.

More specifically, the present invention relates to a positioningcontrol system including a servo-error signal generating circuit, whichcan generate servo-error signal in accordance with a detection currentoutput from at least one photo-detector for detecting the return opticalbeam reflected from a magneto-optical disk device, etc., so as toaccurately irradiate a given optical beam to the desired track position(just focus position), etc., for recording/reproducing operations.

2. Description of the Related Art

In general, in a magneto-optical disk device, an optical disk device(sometimes including a magneto-optical disk device), or the like,recording/reproducing operations corresponding to write/read operationscan be easily performed by utilizing an optical beam without necessityfor complex mechanical elements such as a magnetic head, unlike amagnetic disk drive, a magnetic tape apparatus, or the like. Therefore,the former magneto-optical disk device, etc., can have a larger storagecapacity than the latter magnetic disk device, etc. However, themagneto-optical disk device, etc., is likely to have a difficulty inpositioning a given

In regard to the positioning control system of such an optical beam, twokinds of control operations are performed: one is a tracking servocontrol which enables the track following operations on an opticallyrecording medium such a magneto-optical disk to be performed; and theother is a focusing servo control which enables a focusing position ofthe optical beam on the optically recording medium to be determined.

These control operations are executed in the following sequences:

First, the return optical beam, which is reflected from the opticallyrecording medium, is detected by a plurality of photo-detectors intwo-divisional configurations or in four-divisional configurations; and

Second, a tracking servo-error signal used for tracking servo controland a focusing servo-error signal used for focusing servo control aresimultaneously taken out by utilizing the respective photo-detectors.

In this case, to assure the servo control operations in themagneto-optical disk device, etc., by utilizing an optical beam only, itis necessary to provide a servo-error signal generating circuit whichgenerates each of the above-mentioned servo-error signals adjusted tohave a constant amplitude. Such an adjustment of each of the servo-errorsignals is usually achieved by executing an automatic gain control (AGC)for the servo-error signals on the basis of the detection current outputfrom each of the photo-detectors. Such a servo-error signal generatingcircuit including AGC circuit portion is generally constituted by a lotof electronic components, such as transistors and resistors. Therefore,a positioning control system having the servo-error signal generatingcircuit in a magneto-optical disk device, e.g., is likely to be large insize and expensive. To avoid this disadvantage, these electroniccomponents are desired to be incorporated into an integrated circuit(IC), which can provide a desired circuit with a small size and inrelatively low cost for fabrication.

Here, to enable a problem regarding a conventional positioning controlsystem utilizing an optical beam to be understood more clearly, aconcrete configuration of a positioning control system according to aprior art will be described with reference to the related drawings ofFIGS. 1(A), 1(B), 1(C), 2 and 3. In this case, a positioning controlsystem in a magneto-optical disk device will be illustratedrepresentatively.

FIGS. 1(A), 1(B) and 1(C) are diagrams for explaining servo controloperations for positioning control in a magneto-optical disk device. Tobe more specific, FIG. 1(A) is a perspective view schematically showingthe whole construction of a magneto-optical disk device; FIG. 1(B) is afront view of each of photo-detectors of two-divisional type; and FIG.1(C) is a front view of a photo-detector of four-divisional type.

In FIG. 1(A), 10 denotes a magneto-optical disk 10 which functions as anoptically recording medium and which is rotated by a spindle motor (notshown). Further, 2 denotes an optical head 2 which irradiates an opticalbeam to the predetermined position on the magneto-optical disk 10 forrecording/reproducing operations. The optical head 2 is adapted to bedriven by another motor (not shown) so as to move with respect to theradial direction of the magneto-optical disk 10, and to perform a seekoperation for the predetermined track position.

In regard to the configuration on the surface of the magneto-opticaldisk 10, a plurality of tracks for recording information are formedbetween adjoining guide grooves. Further, both of a trackingposition-error and a focusing position-error are obtained by utilizingthe reflected optical beam, which is reflected from the magneto-opticaldisk 10 and goes back to an optical head 2 when an optical beam of theoptical head 2 is irradiated to the magneto-optical disk 10.

To be more specific, a tracking position-error signal TES is generatedon the basis of the reflected optical beam, and the trackingposition-error of the optical beam corresponding to such a trackingservo-error signal TES is output. Further, taking into consideration thetracking position-error, a track control circuit (not shown) is adaptedto control the optical head 2 to accurately irradiate the predeterminedtrack for recording/reproducing operations.

In a similar manner, a focusing servo-error signal FES is generated onthe basis of the reflected optical beam, and the focusing position-errorof the optical beam corresponding to such a focusing servo-error signalFES is output. Further, taking into consideration the focusingposition-error, a focusing control circuit (not shown) is adapted tocontrol the optical head 2 so that a focus of the optical beam canaccurately conform to the desired track position.

Further, in the optical head 2, the beam emitted from a semiconductorlaser 20 is collected by a collimator lens 21. Then, the collected beamis reflected by a reflection mirror 23, through a beam-splitting prism22 for splitting the beam into a polarized beam, and collected by anobjective (also referred to as an actuator) 24, and irradiated to thesurface of the magneto-optical disk 10,.

At this time, the magnetizing force is given to the irradiated track ofthe magneto-optical disk 10 by a magnet 30 which is arranged confrontingwith the magneto-optical disk 10, and write/read/erase operations on thepredetermined track of the magneto-optical disk 10 can be performed.

Further, the reflected optical beam from the magneto-optical disk 10 iscollected by the objective 24, and reflected by the beam-splitting prism22, through the reflection mirror 23. Further, the reflected opticalbeam from the prism 22 is collected by another collimator lens 26,through a half-wavelength plate 25, and irradiated to both of a trackingphoto-detector 29 of two-divisional type and focusing photo-detector 28of two-divisional type, through another beam-splitting prism 22.

The tracking photo-detector 29 of two-divisional type are constituted bytwo divided portions so that the moving amount of the optical beam (thehatched portion in FIG. 1(B)) with respect to the radial direction ofthe track on the surface of the magneto-optical disk 10 can be detected,as shown in FIG. 1(B). Further, in FIGS. 1(A) and 1(B), the respectiveoutput terminals of the two divided portions of the trackingphoto-detector 29 are connected to a tracking servo-error signalgenerating circuit 4a. Further, the tracking servo-error signalgenerating circuit 4a calculates the value of difference between therespective detection levels A, B of the two output terminals of thetracking photo-detector 29, and consequently outputs the trackingservo-error signal TES corresponding to this calculated value ofdifference.

Further, as shown in FIG. 1(B), the focusing photo-detector 28 oftwo-divisional type are also constituted by two divided portions. Therespective output terminals of the two divided portions of the focusingtracking photo-detector 28 are connected to a focusing servo-errorsignal generating circuit 4b. Further, the focusing servo-error signalgenerating circuit 4b calculates the value of difference between therespective detection levels C, D of the two output terminals of thefocusing photo-detector 29, and consequently outputs the focusingservo-error signal FES corresponding to this calculated value ofdifference.

As described above, in the example of FIG. 1(B), two kinds ofphoto-detectors 28, 29 of two-divisional type are utilized for detectingthe tracking position-error and the focusing position-error.

However, as shown in FIG. 1(C), it is also possible for onephoto-detector of four-divisional type to be utilized, in place of thetwo photo-detectors of two-divisional type as in FIG. 1(B). In thiscase, by means of only one photo-detector of four-divisional type, thetracking servo-error signal TES can be obtained by calculating(A+C)-(B+D), and also the focusing servo-error signal FES is obtained bycalculating (A+B)-(C+D).

In such servo-error signal generating circuits as represented by theabove-mentioned tracking servo-error signal generating circuit 4a andfocusing servo-error signal generating circuit 4b, the respectiveoptical powers of the optical beams utilized in write operations, readoperations and erase operations for the magneto-optical disk 10 areusually different from each other. Accordingly, the differentphoto-current, i.e., different detection current, flows in each of thephoto-detectors 28, 29 at every write/read/erase operation, inaccordance with the the corresponding optical power of the optical beam.Furthermore, the medium reflection factors of the individual tracks onthe magneto-optical disk 10 are also different from each other.Therefore, to obtain assuredly the above-mentioned servo-error signalseach adjusted to have a constant amplitude, it is necessary for AGC forthe servo-error signals to be executed in the servo-error signalgenerating circuit.

To perform such an AGC assuredly, the following AGC operation must beusually executed for the detection currents detected by thephoto-detectors.

First, two kinds of detection currents from each of the photo-detectorsare added together, and then a sum signal is obtained.

Second, one kind of the detection current from each of thephoto-detectors is subtracted from another kind of the detection currenttherefrom, and then a difference signal is obtained.

Third, the above-mentioned sum signal is divided by the above-mentioneddifference signal.

Consequently, both of the tracking servo-error signal and focusingservo-error signal, each having a constant amplitude, can be alwaysobtained.

FIG. 2 is a circuit diagram showing a servo-error signal generatingcircuit of the prior art in the case where a photo-detector oftwo-divisional type is utilized. In this case, either one of thetracking servo-error signal generating circuit and focusing servo-errorsignal generating circuit will be illustrated representatively.

In FIG. 2, resistors R1, R2 are connected to the respectivelycorresponding photo-detector units P1, P2 in the photo-detector oftwo-divisional type. Further, each detection current is converted to thecorresponding voltage (detection voltage), and the thus convertedvoltage is input to each base of two transistors T5, T6. Further, thesevoltages are added together by an operational amplifier 11, and alevel-shift operation for a sum of these voltages is performed by anoperational amplifier 13. Further, the sum of the voltages is input tothe common base of two transistors T2, T3 in a multiplier circuit ofGilbert type.

In this multiplier circuit, the other two transistors T1, T4 areprovided, the respective bases of which bias voltages of direct currenttype (also referred to as d.c. bias voltages) VR are applied to, and therespective collectors of which are connected to a power supply of thevoltage Vcc, via resistors R5, R6. Further, the respective collectors ofthe transistors T2, T3 are coupled together and directly connected tothe power supply of the voltage Vcc as the common collector. An emitterof the transistor T2 is connected to an emitter of the transistor T1 asthe common emitter, while an emitter of the transistor T3 is connectedto an emitter of the transistor T4 as the common emitter. Further, therespective emitters of drive transistors T5, T6 are connected to thecorresponding constant current source 5a, 5b, via resistors R7, R8.

Between a node of the resistor R7 and the constant current source 5a andanother node of the resistor R8 and the constant current source 5b, aresistor R9 is connected. This resistor R9 is adapted to convert thedifferential voltage X-Y as mentioned below to the correspondingdifferential current which flows through the drive transistors T5, T6.

In this case, when the detection voltages of the two photo-detectorunits P1, P2, is respectively X, Y, corresponding to the detectionlevels A, B, the voltage of the collector of the transistor T1 becomes{(X+Y)-X}/(X+Y), i.e., Y/(X+Y). On the other hand, the voltage of thecollector of the transistor T4 becomes {(X+Y)-Y}/(X+Y), i.e., X/(X+Y).Therefore, the difference between the respective collectors of thetransistors T1, T4 becomes (X-Y)/(X+Y); Namely, the difference signalcorresponding to the value of X-Y is divided by the sum signalcorresponding to the value of X+Y. Further, if the difference(X-Y)/(X+Y) is input to an operational amplifier 12 to accuratelycalculate the difference between the respective collectors of thetransistors T1, T4, the servo-error signal (TES or FES), in which theAGC operation as described before has been performed, can be finallyobtained.

Here, Ic1 and VBE1 are assumed to be the collector current of thetransistor T1 and the voltage between the base and the emitter thereof,respectively. Further, Ic2 and VBE2 are assumed to be the collectorcurrent of the transistor T2 and the voltage between the base and theemitter thereof, respectively. Further, Ic5 is assumed to be thecollector current of the transistor T5. By these assumptions, thefollowing equations are obtained.

    Ic1=Is×{exp(q*VBE1/kT)-1}

    Ic2=Is×{exp(q*VBE2/kT)-1}

    Ic5=Ic1+Ic2

Where, q denotes an electric charge of an electron, T denotes anabsolute temperature, k denotes a Boltzmann constant, and Is denotes asaturation current of a collector in the reverse direction.

Further, in the case where the condition represented by exp(q*VBE1/kT)>1and exp(q*VBE1/kT)>1 is satisfied, the following equation is obtained.

    Ic1≠Ic5/[1+exp {q(VBE2-VBE1)/kT}]

Further, the respective currents detected by the photo-detectors P1, P2are logarithmically converted by the operational amplifier 11, and thensupplied to the common base of the transistors T2, T3 as the basevoltage thereof. Therefore, the following equation is further obtained.

    Ic1=Ic5/{1+K*(X+Y)}

Where, K denotes a proportional constant.

Further, in the case where the voltage VR is selected so that thecondition represented by K*(X+Y)>1 can be satisfied, the followingequations are simultaneously obtained.

    Ic1≠Ic5/K*(X+Y)

    Ic4≠Ic6/K*(X+Y)

Where, Ic6 denotes the collector current of the transistor T6.

In this case, since Ic5 and Ic6 correspond to the respective outputcurrents of a differential amplifier having the transistors T5, T6, itis apparent from the above-mentioned equations that Ic1-Ic4 isproportional to (X-Y)/(X+Y).

Further, the collector currents Ic5, Ic6 are converted to thecorresponding voltages by the resistors R5, R6, respectively.Thereafter, these voltages are input to the operational amplifier 12.Finally, the differential signal taken out from the operationalamplifier 12 can be the tracking servo-error signal TES or the focusingservo-error signal FES, which has a constant amplitude independent ofthe change of the amount of optical beam.

FIG. 3 is a circuit diagram showing a servo-error signal generatingcircuit of the prior art in the case where a photo-detector offour-divisional type is utilized. In this case, only the focusingservo-error signal generating circuit will be illustrated as therepresentative of the servo-error signal generating circuits.Hereinafter, any component that is the same as that mentioned beforewill be referred to using the same reference number.

In FIG. 3, resistors R41, R42, R43 and R44 are connected to therespectively corresponding photo-detector units P41, P42, P43 and P44 inthe photo-detector of four-divisional type. Further, the detectioncurrents of the respective photo-detector units P41, P42, P43 and P44are respectively converted to the detection voltages A4, B4, C4 and D4,corresponding to the detection levels A, B, C and D illustrated in FIG.1(C).

The two detection voltages A4, B4 are added together by an operationalamplifier 14 including the predetermined external resistors r1, r2 andr3. On the other hand, the remaining two detection voltages C4, D4 arealso added together by an operational amplifier 15 including thepredetermined external resistors r4, r5 and r6. Two kinds of sums A4+B4and C4+D4 respectively taken out from the amplifiers 15, 16 are input tothe corresponding bases of two transistors T5, T6 in a multipliercircuit having the same circuit configuration as in FIG. 2. Further, thefour detection voltages A4, B4, C4 and D4 are added together by anoperational amplifier 11 including the predetermined external resistorsr7, r8, r9, r10 and r11, and a level-shift operation for a sumA4+B4+C4+D4 of all these voltages is performed by an operationalamplifier 13 including the predetermined external resistors r12, r13,r14 and r15. Further, the sum A4+B4+C4+D4 is input to the common base oftwo transistors T2, T3 in the above-mentioned multiplier circuit.

Further, in a similar manner to the case of FIG. 2, the voltage of thecollector of the transistor T1 becomes{(A4+B4+C4+D4)-(A4+B4)}/(A4+B4+C4+D4), i.e., (C4+D4)/(A4+B4+C4+D4). Onthe other hand, the voltage of the collector of the transistor T4becomes {(A4+B4+C4+D4)-(C4+D4)}/(A4+B4+C4+D4), i.e.,(A4+B4)/(A4+B4+C4+D4). Therefore, the difference between the respectivecollectors of the transistors T1, T4 becomes {(A4+B4)-(C4+D4)}/(A4+B4+C4+D4); Namely, the difference signal corresponding to thevalue of (A4+B4)-(C4+D4) is divided by the sum signal corresponding tothe value of (A4+B4+C4+D4). Further, if the difference{(A4+B4)-(C4+D4)}/(A4+B4+C4+D4) is input to an operational amplifier 12to accurately calculate the difference between the respective collectorsof the transistors T1, T4, the focusing servo-error signal FES, in whichthe AGC operation as described before has been performed, can be finallyobtained.

Further, if other operational amplifiers and another multiplier circuit,respectively having the same configuration as the above-mentionedamplifiers 12, 14 and 15 and the above-mentioned multiplier circuit ofGilbert type, are provided, it becomes also possible for the trackingservo-error signal TES to be obtained.

However, in the positioning control system including the servo-errorsignal generating circuit according to the prior art as illustrated inFIG. 2 or FIG. 3, the following problems have occurred.

First, such a servo-error signal generating circuit is mainlyconstituted by an AGC circuit of the voltage-operation type, in whichthe detection currents in the photo-detectors have to be converted tothe voltage utilized as the servo-error signal. Therefore, it becomesnecessary for the circuit to have a lot of resistors and a lot ofoperational amplifiers in converting the detection currents to theuseful voltages. Consequently, the circuit per se is likely to have thecomplicated configuration, and to be easily affected by external noises.

Second, the circuit has a lot of transistors connected in multi-stageconfiguration. Therefore, it becomes difficult for the levels of all thetransistors to be normally distributed, and also it becomes difficultfor the operating points of all the transistors to be accuratelydetermined. Consequently, all the transistors can not be driven by asingle power supply.

Third, it is difficult for the circuit including such operationalamplifiers and the related resistors to be realized by an IC, which canprovide the desired circuit with a small size and in relatively low costfor fabrication.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, the main object of the presentinvention is to provide a positioning control system utilizing anoptical beam which includes a servo-error signal generating circuithaving simpler circuit configuration than that in the prior art.

A further object of the present invention is to provide a positioningcontrol system utilizing an optical beam which includes a servo-errorsignal generating circuit that is not affected by external noises andthat can utilize a single power supply.

A still further object of the present invention is to provide apositioning control system utilizing an optical beam which can beincorporated into an IC that realize the desired circuit with a smallsize and in relatively low cost for fabrication.

A still further object of the present invention is to provide apositioning control system utilizing an optical beam which can beapplied to a magneto-optical disk device having an optical head and amagneto-optical disk for recording/reproducing operations.

A still further object of the present invention is to provide apositioning control system utilizing an optical beam which can beapplied to an optical disk device having an optical head and an opticaldisk for recording/reproducing operations.

To attain the above objects, the positioning control system utilizing anoptical beam according to the present invention has a photo-detectorconstituted by at least two-divisional photo-detector units; and aservo-error signal generating circuit which can generate at least oneservo-error signal for servo control of the optical beam in accordancewith a difference between the respective detection currents detected bythe photo-detector units.

Further, the servo-error signal generating circuit includes at least onedivision circuit that has two pairs of transistors, two emitters in eachpair of transistors being connected together into a common emitter, d.c.bias voltages being applied to the respective bases corresponding totransistors on one side in the respective pairs of transistors, therespective collectors corresponding to the transistors on one side beingconnected to a common connecting portion via resistors, the commonconnecting portion being connected to a power supply, the respectivebases corresponding to transistors on the other side being connectedtogether into a common base.

Further, an integrating capacitor is connected to the common basecorresponding to the transistors on the other side, and integrates adifference between the current equivalent to the current flowing throughthe above-mentioned common connecting portion and the predeterminedreference current, to apply the thus integrated difference to the commonbase of the transistors on the other side.

Further, the photo-detector units are respectively connected to thecommon emitters of the respective pairs of the transistors, so that therespective detection currents can flow through the common emitters.

Further, the division circuit is adapted to obtain the servo-errorsignal from the respective potentials of the respective collectors ofthe transistors on one side.

Preferably, the common connecting portion is connected to the powersupply via a current-mirror circuit. The integrating capacitor isoperative to integrate a difference between the mirror current in thecurrent-mirror circuit and the reference current.

In such a construction, since the detection currents detected by therespective photo-detectors can be directly input to the divisioncircuit, it is not necessary for a lot of resistors and operationalamplifiers to be provided. Therefore, the servo-error signal generatingcircuit in the positioning control system can be realized by anintegrated circuit.

Further, preferably, the photo-detector units are connected to thecommon emitters in the respective pairs of the transistors via aplurality of current-mirror circuits.

In the case where four-divisional photo-detector units are connected toa plurality of division circuits, the division circuits are operative toobtain both of a tracking servo-error signal for tracking servo controland a focusing servo-error signal for focusing servo control.

In an alternative preferred embodiment, the positioning control systemaccording to the present invention includes a servo-error signalgenerating circuit which has two pairs of transistors, two bases in eachpair of the two pairs of transistors being connected together into acommon base, the respective collectors corresponding to the transistorson one side in the respective pairs of transistors being connected to acommon connecting portion via corresponding resistors, the commonconnecting portion being connected to a power supply, the respectiveemitters corresponding to the transistors on one side being connectedtogether to a constant current source.

Further, at least two photo-detector units are connected to therespective collectors corresponding to transistors on the other side, sothat the respective detection currents can flow through the respectivecollectors corresponding to the transistors on the other side, and therespective emitters corresponding to the transistors on the other sideare connected together into a common emitter.

In this case, the positioning control system is adapted to control anamplitude of the servo-error signal on the basis of the currentgenerated in the constant current source. By virtue of such a constantcurrent source, the positioning control system can be realized withoutan integrating capacitor for preventing the system from causing anoscillation. Therefore, it can be easier to realize the servo-errorsignal generating circuit by an integrated circuit.

In another alternative preferred embodiment, a servo-error signalgenerating circuit includes two pairs of current-mirror circuits inwhich the respective bases of a plurality of first transistors and therespective bases of a plurality of second transistors are connectedtogether, and a base and a collector of each of the first transistorsare connected together; a first voltage source which is connected incommon to the respective emitters of the first transistors; a currentsource which is connected in common to the respective emitters of thesecond transistors; two-divisional photo-detector units which have firstterminals connected in common to a second voltage source, and havesecond terminals connected to the respective collectors of the firsttransistors, to detect the return optical beam; and two resistors whichhave first terminals connected to the respective collectors of thesecond transistors, and have second terminals connected in common to asecond voltage source.

Typically, the positioning control system according to the presentinvention can applied to a magneto-optical disk device, which includes amagneto-optical disk that functions as the optically recording mediumand an optical head which irradiates the optical beam to thepredetermined position on the magneto-optical disk.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and features of the present invention will be moreapparent from the following description of the preferred embodimentswith reference to the accompanying drawings, wherein:

FIGS. 1(A), 1(B) and 1(C) are diagrams for explaining servo controloperations for positioning control in a magneto-optical disk device;

FIG. 2 is a circuit diagram showing a servo-error signal generatingcircuit of the prior art in the case where a photo-detector oftwo-divisional type is utilized;

FIG. 3 is a circuit diagram showing a servo-error signal generatingcircuit of the prior art in the case where a photo-detector offour-divisional type is utilized;

FIGS. 4(A) and 4(B) are essential embodiments each based on theprinciple of the present invention;

FIG. 5 is a circuit diagram showing a first preferred embodimentaccording to the present invention;

FIG. 6 is a circuit diagram showing a second preferred embodimentaccording to the present invention;

FIG. 7 is a circuit diagram showing a third preferred embodimentaccording to the present invention;

FIG. 8 is a circuit diagram showing a fourth preferred embodimentaccording to the present invention;

FIG. 9 is a circuit diagram showing a fifth preferred embodimentaccording to the present invention;

FIG. 10 is a circuit diagram showing a sixth preferred embodimentaccording to the present invention;

FIG. 11 is a circuit diagram showing a seventh preferred embodimentaccording to the present invention;

FIG. 12 is a circuit diagram showing a modification of the seventhpreferred embodiment illustrated in FIG. 11;

FIG. 13 is a circuit diagram showing an eighth preferred embodimentaccording to the present invention;

FIG. 14 is a circuit diagram showing a ninth preferred embodimentaccording to the present invention; and

FIG. 15 is a circuit diagram showing a modification of the ninthpreferred embodiment illustrated in FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 4(A) and 4(B) are circuits each showing an essential embodiment ofa positioning control system based on the principle of the presentinvention. To be more specific, FIG. 4(A) shows an embodiment based onthe first principle; and FIG. 4(B) shows another embodiment based on thesecond principle. In this case, the servo-error signal generatingcircuit, which is the main part of a positioning control system of thepresent invention is illustrated representatively to simplify theexplanation.

Hereinafter, it should be noted that the positioning control systemincludes both of the tracking servo control and the focusing servocontrol as described before.

In FIG. 4(A), a photo-detector is constituted by two-divisionalphoto-detector units P1, P2 for detecting the return optical beamreflected from a recording medium (not shown). Further, a servo-errorsignal generating circuit is provided, which can generate at least oneservo-error signal for servo control of the optical beam in accordancewith a difference between the respective detection currents detected bythe photo-detector units P1, P2, so as to accurately irradiate anoriginal optical beam to the desired position on the basis of theservo-error signal.

Such a servo-error signal generating circuit includes a division circuitthat has two pairs of transistors Q1˜Q4. Two emitters in each pair ofthe two pairs of transistors Q1˜Q4 are connected together into a commonemitter. Further, d.c. bias voltages (e.g., 3 V) are applied to therespective bases corresponding to transistors Q2, Q4 on one side in therespective pairs of transistors Q1˜Q4. Further, the respectivecollectors corresponding to the transistors Q2, Q4 on one side in therespective pairs of transistors Q1˜Q4 are connected to the correspondingresistors R5, R6 through which the currents IQ2, IQ4 respectively flow,and coupled together into a common connecting portion so as to connectto a power supply of the voltage Vcc. Further, the respective basescorresponding to transistors Q1, Q3 on the other side in the respectivepairs of transistors, through which the currents IQ1, IQ3 respectivelyflow, are connected together into a common base.

Further, an integrating capacitor C is connected to the common basecorresponding to the transistors Q1, Q3 on the other side. Theintegrating capacitor C is adapted to integrate a difference between thecurrent equivalent to the current flowing through the common connectingportion of the respective collectors and the predetermined referencecurrent Iref generated by a constant current source 5, to apply the thusintegrated difference to the common base corresponding to thetransistors Q1, Q3 on the other side.

Further, the photo-detector units P1, P2 are respectively connected tothe common emitters in the respective pairs of the transistors Q1˜Q4, sothat the respective detection currents can flow through the commonemitters. Further, the division circuit takes out the servo-error signalas the output voltage Vout from the respective potentials of therespective collectors of the transistors Q2, Q4 on one side.

Further, in FIG. 4(A), the common connecting portion of the respectivecollectors of the transistors Q2, Q4 is connected to the power supplyvia a current-mirror circuit 6. The integrating capacitor C is operativeto integrate a difference between the mirror current generated in thecurrent-mirror circuit 6 and the reference current Iref, and to executea feedback operation by applying the thus integrated difference to thecommon base of the transistors Q1, Q3 on the other side.

In conceiving the essential embodiment according to the presentinvention as represented by FIG. 1(A), a special attention was paid tothe disadvantage of the prior art that the servo-error signal generatingcircuit is mainly constituted by an AGC circuit of the voltage-operationtype and requires a lot of resistors and operational amplifiers. To copewith this disadvantage, an AGC circuit of current operation mode, whichhas the simpler construction, easily realized by an IC, and not affectedby external noises, is utilized for the present invention.

To be more concrete, the sequences for obtaining the servo-error signalin FIG. 4(A) are as follows:

First, the difference between the current equivalent to the currentflowing through the common connecting portion and the reference currentIref is integrated and converted to the corresponding voltage by theintegrating capacitor C;

Second, this voltage is input to the common base of the transistors Q1,Q3 on the other side by a feedback operation;

Third, the detection currents respectively flow through thephoto-detector units P1, P2 which are connected to the common emittersin the respective pairs of the transistors Q1˜Q4; and

Fourthly, the output voltage Vout is finally taken out from therespective potentials of the respective collectors of the transistorsQ2, Q4 on one side, as the servo-error signal of the voltage(X-Y)/(X+Y).

In such sequences, an AGC circuit of current operation mode, in which alot of resistors, etc., for converting the detection currents to thedetection voltages need not be provided, is utilized for an AGCoperation. Therefore, the circuit configuration becomes simpler than inthe prior art, and it becomes possible for the AGC circuit to berealized by an IC.

Further, in the case where the common connecting portion of therespective collectors of the transistors Q2, Q4 is connected to thepower supply via a current-mirror circuit 6, the integrating capacitor Cintegrates a difference between the mirror current in the current-mirrorcircuit and the reference current Iref, and converts the difference tothe corresponding voltage. In such a construction, this voltageproportional to X+Y can be input to the common base of the transistorsQ1, Q3 by a feedback operation, without a loss of electric power due toresistors.

In FIG. 4(B), a photo-detector is also constituted by two-divisionalphoto-detector units P1, P2, similar to the case of FIG. 4(A). Further,a servo-error signal generating circuit is also provided, which cangenerate at least one servo-error signal for servo control of theoptical beam in accordance with a difference between the respectivedetection currents detected by the photo-detector units P1, P2.

Such a servo-error signal generating circuit also includes two pairs 8a,8b of transistors Q1˜Q4. Further, two bases in each pair of the twopairs 8a, 8b of transistors Q1˜Q4 are connected together into a commonbase. Further, the respective collectors corresponding to thetransistors Q2, Q3 on one side in the respective pairs 8a, 8b oftransistors Q1˜Q4, through the currents IQ2, IQ3 respectively flow, areconnected to the corresponding resistors R5, R6 and coupled togetherinto a common connecting portion so as to connect to a power supply ofthe voltage Vcc. Further, the respective emitters of the transistors Q2,Q3 on one side are connected together to a constant current source 5.

Further, the photo-detector units P1, P2 are connected to the respectivecollectors corresponding to transistors Q1, Q4 on the other side in therespective pairs 8a, 8b of transistors Q1˜Q4, so that the respectivedetection currents IQ1, IQ4 can flow through the respective collectorsof the transistors Q1, Q4 on the other side. Further, the respectiveemitters of the transistors Q1, Q4 on one other side are connectedtogether into a common emitter. Further, the servo-error signalgenerating circuit takes out the servo-error signal from the respectivepotentials of the respective collectors of the transistors Q2, Q3 on oneside, similar to the case of FIG. 4(B).

In this case, the positioning control system is operative to control anamplitude of the servo-error signal on the basis of the currentgenerated in the constant current source 5. Namely, an AGC circuit ofcurrent operation mode, which is not affected by external noises, isalso utilized, in place of an AGC circuit of voltage operation mode asin the prior art.

To be more concrete, the sequences for obtaining the servo-error signalin FIG. 4(B) are as follows:

First, the detection currents respectively flow through thephoto-detector units P1, P2 which are connected to the respectivecollectors of the transistors Q1, Q4 on the other side; and

Second, the output voltage Vout is taken out from the respectivepotentials of the respective collectors of the transistors Q2, Q3 on oneside, as the servo-error signal of the voltage (X-Y)/(X+Y).

In this case, since the feedback operation for the bases of transistorsas described in FIG. 4(A) is not executed, the frequency characteristicsof the positioning control system is not deteriorated due to such afeedback operation. Therefore, an integrating capacitor for stabilizingthe system (shown in FIG. 4(A)) is not necessary. Consequently, theservo-error generating circuit in FIG. 4(B) becomes simpler than that inFIG. 4(A), and can be constituted more easily than that in FIG. 4(A).

FIG. 5 is a circuit diagram showing a first preferred embodimentaccording to the present invention. In this case, a servo-error signalgenerating circuit, which is the main part of a positioning controlsystem of the present invention, will be explained more concretely thanin FIG. 4(A).

In FIG. 5, similar to the case of FIG. 4(A), a servo-error signalgenerating circuit includes a division circuit that has two pairs oftransistors Q1˜Q4. Two emitters in each pair of the two pairs oftransistors Q1˜Q4 are connected together into a common emitter. Further,d.c. bias voltages (e.g., 3 V) are applied to the respective basescorresponding to transistors Q2, Q4 on one side in the respective pairsof transistors Q1˜Q4.

Further, the respective collectors corresponding to the transistors Q2,Q4 on one side in the respective pairs of transistors Q1˜Q4 areconnected to the corresponding resistors R5, R6 which have the sameresistance values each other and through which the currents IQ2, IQ4respectively flow. Further, the respective ends of resistors R5, R6 arecoupled together into a common connecting portion and connected to apower supply of the voltage Vcc, via a current-mirror circuit 6.

A difference between the mirror current generated in the current-mirrorcircuit 6 and the reference current Iref is integrated by an integratingcapacitor C, and the thus integrated value is input to the common baseof the transistors Q1, Q3 on the other side to execute a feedbackoperation.

Here, such an integrating capacitor C is provided to perform a phasecompensation of a feedback loop of the circuit and to prevent thecircuit from causing an oscillation.

Further, an operational amplifier 12 is connected to the respectivecollectors of the transistors Q2, Q4 on one side. The operationalamplifier 12 is operative to calculate a differential output voltageVout between the respective potentials of the respective collectors ofthe transistors Q2, Q4 and to output the tracking servo-error signal TESor the tracking servo-error signal FES. Further, the photo-detectors oftwo-divisional type (corresponding to the tracking photo-detector 29 orthe focusing photo-detector 28 shown in FIG. 1(A)) is constituted by twophoto-detector units P1, P2. The respective anodes of thesephoto-detector units P1, P2 are connected to the ground, while therespective cathodes thereof are connected to the common emitters of therespective pairs of transistors Q1˜Q4.

Further, the operation of the servo-error signal generating circuit inFIG. 5 will be described hereinafter.

Here, VBE is assumed to be the voltage between a base and an emitter ofa transistor, and Ic is assumed to be the collector current. Therelation between VBE and Ic is expressed by the following equation (1).

    Ic=Is×{exp(q*VBE/kT)-1}                              (1)

Where, q denotes an electric charge of an electron, T denotes anabsolute temperature, k denotes a Boltzmann constant, and Is denotes asaturation current of a collector in the reverse direction, as describedbefore.

At the room temperature (e.g., 300° K.), if VBE is 0.1 V, the value ofthe term of an exponential function becomes 47.7; if VBE is 0.2 V, thevalue becomes 227.4; and if VBE is 0.6 V, the value becomes11,700,000,000. Therefore, the condition represented by exp(q*VBE1/kT)>1is satisfied, and the equation (1) is approximately expressed by thefollowing equation (2).

    Ic≠Is×{exp(q*VBE/kT)}                          (2)

When the equation (2) is applied to the circuit shown in FIG. 5, therespective collector currents IQ1, IQ2, IQ3 and IQ4 of the transistorsQ1, Q2 Q3 and Q4 are expressed by the following equations (3A)˜(3D).

    IQ1=Is×{exp(q*VBE1/kT)}                              (3A)

    IQ2=Is×{exp(q*VBE2/kT)}                              (3B)

    IQ3=Is×{exp(q*VBE3/kT)}                              (3C)

    IQ4=Is×{exp(q*VBE4/kT)}                              (3D)

When a ratio of IQ1 with respect to IQ2, corresponding to the ratiobetween the respective collector currents in the collectors Q1, Q2 inwhich the emitters are connected together, is taken, the followingequation (4) is obtained.

    IQ1/IQ2=exp {q(VBE1-VBE2)/kT}]                             (4)

In a similar manner, When a ratio of IQ3 with respect to IQ4,corresponding to the ratio between the respective collector currents inthe collectors Q3, Q4 in which the emitters are connected together, istaken, the following equation (5) is obtained.

    IQ3/IQ4=exp {q(VBE3-VBE4)/kT}]                             (5)

Since the value of (VBE1-VBE2) is equal to that of (VBE3-VBE4), i.e.,VBE1-VBE2=VBE3-VBE4, the equation (4) conforms to the equation (5).Namely, the following equation (6) is obtained.

    IQ1/IQ2=IQ3/IQ4=α                                    (6)

Here, it is assumed that the feedback operation in the feedback loop isexecuted so that the value of (IQ2+IQ4) is equal to Iref, i.e.,IQ2+IQ4=Iref.

Further, it is assumed that the following two relations are satisfied.

    IQ1+IQ2=X

    IQ3+IQ4=Y

In this case, the value of (X-Y)/(X+Y) corresponding to the servo-errorsignal is expressed by the following equation (7).

    (X-Y)/(X+Y)={(IQ1+IQ2)-(IQ3+IQ4)}/(IQ1+IQ2+IQ3+IQ4)        (7)

When the equation (6) is substituted into the equation (7) to eliminateIQ1, IQ3 from the latter equation (7), the following equation (8) isobtained. ##EQU1##

Since the value of R5 is equal to that of R6, i.e., R5=R6=R, the outputvoltage Vout corresponding to the difference between the respectivecollectors of the transistors Q2, Q4 is expressed by the followingequation (9).

    Vout=R×Iref×(X-Y)/(X+Y)                        (9)

Therefore, if an AGC operation is performed in current mode, theservo-error signal (tracking servo-error signal or focusing servo-errorsignal), corresponding to the difference between the detection currentsin the photo-detector units P1, P2, can be generated assuredly.

The servo-error generating circuit including an AGC circuit of currentoperation mode is not affected by the external noises. Further, theabove-mentioned circuit has the relatively simple configuration, andrequires few stages of transistors and a small number of operationalamplifier. Accordingly, the circuit can be driven by a single powersupply. Therefore, the portion of FIG. 5 surrounded by the broken linecan be realized by an IC with a small size and in low cost forfabrication.

In this case, it should be noted that an integrating capacitor must beattached to the circuit from the outside thereof, to perform the phasecompensation for the feedback loop of the circuit.

FIG. 6 is a circuit diagram showing a second preferred embodimentaccording to the present invention. In this figure, a servo-error signalgenerating circuit, which is the main part of a positioning controlsystem of the present invention, will be mainly illustrated.

In FIG. 6, both cathodes of photo-detector units P1, P2 are connected incommon to a power supply of the voltage Vcc. Further, anodes of thephoto-detector units P1, P2 are connected to a transistor Q7 of acurrent-mirror circuit 7a and a transistor Q9 of a current-mirrorcircuit 7b, respectively. Further, the respective collectors of anothertransistor Q8 of a current-mirror circuit 7a and a transistor Q10 of acurrent-mirror circuit 7b are connected to the respective commonemitters of two pairs of transistors Q1˜Q4 in the division circuit 4 asdescribed in FIG. 5.

In such a circuit, the detection current Ix in the photo-detector unitP1 corresponds to the mirror current Ix' in the transistor Q8 of thecurrent-mirror circuit 7a, while the detection current Iy in thephoto-detector unit P2 corresponds to the mirror current Iy' in thetransistor Q10 of the current-mirror circuit 7b. Therefore, the sameoperation as described in FIG. 5 can be performed, and the desiredservo-error signal can be obtained.

In this case, by virtue of the current-mirror circuits 7a, 7b, thevoltage Vcc of the power supply can be directly applied to thephoto-detector units, respectively. Therefore, the voltage of the powersupply can be utilized more effectively than the case of FIG. 5.Further, in the case where the current-multiplication factor is adjustedto the value that is equal to or more than 1, it becomes possible forthe sensitivity of the photo-detector to be increased.

It should be noted that these current-mirror circuits 7a, 7b can beconstituted by an IC, together with the other circuit portions.

FIG. 7 is a circuit diagram showing a third preferred embodimentaccording to the present invention. Also in this figure, a servo-errorsignal generating circuit will be mainly illustrated.

In FIG. 7, four-divisional photo-detector units are utilized tosimultaneously generate the tracking servo-error signal TES and thefocusing servo-error signal FES. In this case, photo-detector units P1,P2, P3 and P4 are respectively connected to transistors Q7, Q9, Q11 andQ13 for servo control in the respective current-mirror circuit 7a˜7d.

Further, the current-mirror circuits 7a˜7d include the respective pairsof transistors Q8, Q8', Q10, Q10', Q12, Q12', and Q14, Q14' on therespective mirror sides thereof.

Further, a collector of the transistor Q8' of the current-mirror circuit7a and a collector of the transistor Q14' of the current-mirror circuit7d are connected in common to the sides x of the division circuit 4(shown in FIG. 5) for tracking servo control. Further, the transistorQ10' of the current-mirror circuit 7b and the transistor Q12' of thecurrent-mirror circuit 7c are connected in common to the sides y of thedivision circuit 4 for focusing servo control.

Further, a collector of the transistor Q8 of the current-mirror circuit7a and a collector of the transistor Q10 of the current-mirror circuit7b are connected in common to the sides x of the division circuit 4 fortracking servo control. Further, the transistor Q12 of thecurrent-mirror circuit 7c and the transistor Q14 of the current-mirrorcircuit 7d are connected in common to the sides y of the divisioncircuit 4 for focusing servo control.

Two operational amplifiers 12a, 12b are connected to the divisioncircuit 4 for tracking servo control and the other division circuit 4for focusing servo control, respectively. The operational amplifiers12a, 12b output the tracking servo-error signal TES and the focusingservo-error signal FES, respectively.

In such a construction, the division circuit 4 for tracking servocontrol generates two kinds of output voltages corresponding to(A+C)/(A+B+C+D) and (B+D)/(A+B+C+D), with respect to the detectionlevels A˜D respectively corresponding to the detection currents in thephoto-detectors P1˜P4. Further, a difference between the two kinds ofoutput voltages are calculated by the operational amplifier 12a, andfinally the tracking servo-error signal TES, for which an AGC operationis executed, can be obtained.

In a similar manner, the other division circuit 4 for focusing servocontrol generates two kinds of output voltages corresponding to(A+B)/(A+B+C+D) and (C+D)/(A+B+C+D), with respect to the detectionlevels A˜D respectively corresponding to the detection currents in thephoto-detectors P1˜P4. Further, a difference between the two kinds ofoutput voltages are calculated by the operational amplifier 12b, andfinally the focusing servo-error signal FES, for which an AGC operationis executed, can be obtained.

It should be noted that these current-Mirror circuits 7a˜7d can beconstituted by an IC, together with the circuit shown in FIG. 5.

FIG. 8 is a circuit diagram showing a fourth preferred embodimentaccording to the present invention. Also in this figure, a servo-errorsignal generating circuit will be mainly illustrated.

In FIG. 8, emitters of the respective transistors Q2, Q3 on one side oftwo pairs of transistors 8a, 8b are connected in common to a constantcurrent source 5. Further, collectors of the respective transistors Q2,Q3 on one side are connected to a power supply of the voltage Vcc, viaresistors R5, R6 having the same resistance values each other,respectively.

Further, in FIG. 8, emitters of the respective transistors Q1, Q4 on theother side of two pairs of transistors 8a, 8b are connected in common tothe ground. Further, collectors of the respective transistors Q1, Q4 onthe other side are respectively connected to anodes of photo-detectorunits in which both cathodes are connected to the power supply.

Further, the collectors of the respective transistors Q2, Q3 on one sideare also connected to two input terminals of an operational amplifier12, respectively.

Further, the operation of the servo-error signal generating circuit inFIG. 8 will be described hereinafter.

When the equation (3) described in the first preferred embodiment istransformed so that it is expressed in terms of VBE1˜VBE4, the followingequations (10A)˜(10D) are obtained.

    VBE1≠Vt×ln(IQ1/Is)                             (10A)

    VBE2≠Vt×ln(IQ2/Is)                             (10B)

    VBE3≠Vt×ln(IQ3/Is)                             (10C)

    VBE4≠Vt×ln(IQ4/Is)                             (10D)

Where, it is assumed that the condition represented by Vt=kT/q issatisfied.

In these equations (10A)˜(10D), ln(z) means log e(z)., naturallogarithm, where z denotes any positive number.

By examining the relation between two transistors Q1, Q4 which arerespectively connected to the photo-detectors P1, P4, the followingequation (11) is obtained.

    VBE1-VBE4=Vt×ln(IQ1/Is)-Vt×ln(IQ4/Is)          (11)

By utilizing the property of the logarithm, the equation (11) istransformed into the following equation (12).

    VBE1-VBE4=Vt×ln(IQ1/IQ4)                             (12)

In a similar manner, by examining the relation between two transistorsQ2, Q3, the following equation (13) is obtained.

    VBE2-VBE3=Vt×ln(IQ2/Is)-Vt×ln(IQ3/Is)          (13)

By utilizing the property of the logarithm, the equation (13) istransformed into the following equation (14).

    VBE2-VBE3=Vt×ln(IQ2/IQ3)                             (14)

In this case, since the value of (VBE1-VBE4) is equal to that of(VBE2-VBE3), i.e., VBE1-VBE4=VBE2-VBE3, the following equation (15) isobtained.

    IQ1/IQ4=IQ2/IQ3=α                                    (15)

Further, since the transistors Q2, Q3 are both connected to the constantcurrent source 5, the following equation (16) is obtained.

    IQ2+IQ3=Iref=constant                                      (16)

Further, the output voltage Vout corresponding to the difference betweenthe respective collectors of the transistors Q2, Q3 is expressed by thefollowing equation (17).

    Vout=R×(IQ2-IQ3)                                     (17)

In accordance with the equations (15), (16 ), it is confirmed that IQ2is equal to the value of Iref* α/(α+1), i.e., IQ2=Iref*α/(α+1), and thatIQ3 is equal to the value of Iref/(α+1), i.e., IQ3=Iref/(α+1).

By substituting these relations into the equation (17), the followingequation (18) is obtained.

    Vout=R×Iref×(α-1)/(α+1)            (18)

Further, by utilizing the equation (15), it is confirmed that the valueof (α-1) is equal to that of (IQ1/IQ4-1), i.e., (α-1)=IQ1/IQ4-1, andthat the value of (a+1) is equal to that of (IQ1/IQ4+1), i.e.,(α+1)=IQ1/IQ4+1. When these relations are substituted into the equation(18), the following equation (19) is obtained. ##EQU2##

As apparent from the equation (19), the value of quotient, in which adifference between the respective detection currents in thephoto-detector P1, P2 is divided by a sum of the respective detectioncurrents in the photo-detector P1, P2, is obtained as the output voltageVout. This value corresponds to the servo-error signal for which an AGCoperation is performed in current mode.

The servo-error generating circuit including an AGC circuit of currentoperation mode is not affected by the external noises. Further, theabove-mentioned circuit has the relatively simple configuration, andrequires few stages of transistors and a small number of operationalamplifier. Accordingly, the circuit can be driven by a single powersupply. Therefore, the portion of FIG. 8 surrounded by the broken linecan be realized by an IC with a small size and in low cost forfabrication, similar to the case of FIGS. 5 to 7.

In this case, since the circuit has no feedback loop, an integratingcapacitor is not necessary. Therefore, the frequency characteristics ofthe positioning control system is not deteriorated due to the feedbackoperation. Consequently, an operational amplifier 12 has only to beprovided as the additional circuit provided outside the servo-errorsignal generating circuit.

FIG. 9 is a circuit diagram showing a fifth preferred embodimentaccording to the present invention. Also in this figure, a servo-errorsignal generating circuit will be mainly illustrated.

In FIG. 9, four-divisional photo-detector units are utilized tosimultaneously generate the tracking servo-error signal TES and thefocusing servo-error signal FES. In this case, photo-detector units P1,P2, P3 and P4 are respectively connected to transistors Q1, Q3, Q5 andQ7 of the respective pairs of transistors 8a˜8d. In this case, only fourtransistors Q1, Q1', Q2 and Q2' are illustrated, and the other twelvetransistors Q3˜Q8' are omitted, so that the drawing is not complicated.

In FIG. 9, the pairs of transistors 8a˜8d include the respective pairsof transistors Q2, Q2', Q4, Q4', Q6, Q6', and Q8, Q8'. Further, suchpairs of transistors 8a˜8d respectively include the transistors Q1',Q3', Q5' and Q7' for compensating for the respective base currents andthe high-speed diode d1, d3, d5 and d7 (or otherwise d1, d2, d3, d4, d5,d6, d7 and d8 may be provided for all the pairs of transistors).

Further, a collector of the transistor Q2' of the pair of transistors 8aand a collector of the transistor Q6' of the pair of transistors 8c areconnected in common to a resistor R5' for tracking servo control(indicated by IX-T). Further, a collector of the transistor Q4' of thepair of transistors 8b and a collector of the transistor Q8' of the pairof transistors 8d are connected in common to a resistor R6' for trackingservo control (indicated by IY-T).

Further, a collector of the transistor Q2 of the pair of transistors 8aand a collector of the transistor Q4 of the pair of transistors 8b areconnected in common to a resistor R5 for focusing servo control(indicated by IX-F). Further, a collector of the transistor Q6 of thepair of transistors 8c and a collector of the transistor Q8 of the pairof transistors 8d are connected in common to a resistor R6 for trackingservo control (indicated by IY-F).

Further, the resistor R5' and the resistor R6' are connected to twoinput terminals of an operational amplifier 12a for tracking servocontrol, respectively. Further, the resistor R5 and the resistor R6 areconnected to two input terminals of an operational amplifier 12b forfocusing servo control, respectively. The operational amplifiers 12a,12b output the tracking servo-error signal TES and the focusingservo-error signal FES, respectively.

In such a construction, by means of the resistors R5' and the resistorR6' for tracking servo control, two kinds of output voltagescorresponding to (A+C)/(A+B+C+D) and (B+D)/(A+B+C+D) are generated, withrespect to the detection levels A˜D respectively corresponding to thedetection currents in the photo-detectors P1˜P4. Further, a differencebetween the two kinds of output voltages are calculated by theoperational amplifier 12a, and finally the tracking servo-error signalTES, for which an AGC operation is executed, can be obtained.

In a similar manner, by means of the resistor R5 and the resistor R6 forfocusing servo control, two kinds of output voltages corresponding to(A+B)/(A+B+C+D) and (C+D)/(A+B+C+D) are generated, with respect to thedetection levels A˜D respectively corresponding to the detectioncurrents in the photo-detectors P1˜P4. Further, a difference between thetwo kinds of output voltages are calculated by the operational amplifier12b, and finally the focusing servo-error signal FES, for which an AGCoperation is executed, can be obtained.

It should be noted that these pairs of transistors can be constituted byan IC, similar to the case of FIGS. 5 to 8.

FIG. 10 is a circuit diagram showing a sixth preferred embodimentaccording to the present invention. In this figure, an example, in whicha multiplier circuit of Gilbert type (hereinafter referred to as aGilbert-multiplier circuit) according to the present invention isapplied to a servo-error signal generating circuit of themagneto-optical disk device, is illustrated.

In FIG. 10, a servo-error signal generating circuit is adapted togenerate the servo-error signal TES (FES) in accordance with adifference between the respective detection currents detected bytwo-divisional photo-detector units P1, P2 for detecting the returnoptical beam reflected from a magneto-optical disk 10 (shown in FIG.1(A)).

Further, in FIG. 10, the servo-error signal generating circuit isconnected to a Gilbert-multiplier circuit, and the adequately modifiedservo-error signal generating circuit 4 for recording/reproducingoperations of the magneto-optical disk 10 is constituted.

Here, the circuit configuration of such a servo-error signal generatingcircuit 4 will be briefly described. As shown in FIG. 10, the respectiveterminals of the resistors R5, R6 are both connected to a power supplyof the voltages Vcc. Further, the other terminals of the resistors R5,R6 are connected to the respectively corresponding collectors oftransistors Q2, Q3. Further, the respective emitters of transistors Q2,Q3 are connected in common to the same constant current source 5.

The output from one photo-detector units P1 is connected to thecollector of the transistor Q1 in which a short circuit is formedbetween the collector and the base. Further, the emitter of thetransistor Q1 is connected to another power supply of the voltage E. Inthis case, this power supply of the voltage E provides a bias voltagewhich is necessary for the constant current source to be operated and togenerate the constant current Iref.

In a similar manner, the output from one photo-detector units P2 isconnected to the collector of the transistor Q4 in which a short circuitis formed between the collector and the base. Further, the emitter ofthe transistor Q4 is connected to the power supply of the voltage E.

Here, the circuit 8a, 8b are so-called current-mirror circuit which isan important feature of the present invention.

The output signals of the Gilbert-multiplier circuit, which are takenout from the respective nodes A6, B6, are amplified by a differentialamplifier (operational amplifier) 12, and the servo-error signal TES(FES) is obtained.

Further, the operation of the Gilbert-multiplier circuit in FIG. 8 willbe described hereinafter.

The currents flowing through the transistors Q1, Q2, Q3 and Q4 areassumed to be IQ1, IQ2, IQ3 and IQ4, respectively. In thisGilbert-multiplier circuit, the emitters of the transistors Q1, Q4 areconnected together into a common emitter, and the constant voltage E isapplied to the common emitter. Further, the other emitter of thetransistors Q2, Q3 are connected together to the constant current source5.

In the case where the constant current is supplied to theGilbert-multiplier circuit, the conditions similar to the case of FIG. 7is satisfied.

To be more specific, the following relations as described in theequations (10A)˜(10B) is obtained.

    VBE1=Vt×ln(IQ1/Is1)

    VBE2=Vt×ln(IQ2/Is2)

Where, Vt=kT/8.

The voltageΔ VBE generated between the bases of the respectivetransistors Q1, Q4 are expressed by the following equation (20).

    ΔVBE=VBE1-VBE4=Vt×ln(IQ1/Is1) -Vt×ln(IQ4/Is4)(20)

Here, all the transistors Q1˜Q4 are fabricated by the same process andhave the same shape each other, and therefore Is1˜Is4 are all equal,i.e., Is=Is1=Is2=Is3=Is4. The equation (20) is transformed into thefollowing equation (21).

    ΔVBE=Vt×ln(IQ1/IQ4)                            (21)

In a similar manner, the respective voltage Δ VBE generated between thebases of the respective transistors Q2, Q3 are expressed by thefollowing equation (22).

    ΔVBE=Vt×ln(IQ2/IQ3)                            (22)

Since the potentials of the bases of the respective transistors Q1, Q2are equal with each other and also the potentials of the bases of therespective transistors Q3, Q4 are equal with each other, the equation(22) conforms to the equation (22). Therefore, the following equation(23) is obtained.

    IQ1/IQ4=IQ2/IQ3                                            (23)

Here, in the case where IQ1 is equal to IQ4, and IQ2 is equal to IQ3,the equation (23) is valid.

Further, it should be noted that the value of (IQ2 +IQ3) is constant,i.e., IQ2+IQ3=I=constant. The currents IQ2, IQ3 are converted to therespectively corresponding voltages by the resistors R5, R6. Further,two kinds of voltages taken out from the respective resistors R5, R6 areconverted to the output voltage Vout, which is the signal of a singleend, by the differential amplifier 12. Thus, the output voltage Vout isobtained in accordance with the value of quotient, in which a differencebetween the respective detection currents in the photo-detector P1, P2is divided by a sum of the respective detection currents in thephoto-detector P1, P2, with the limitation of a maximum amplitudeIR(R1=R2=R) in the output voltage. Under such a condition, the followingequation (24) is obtained. ##EQU3##

FIG. 11 is a circuit diagrams showing a seventh preferred embodimentaccording to the present invention; and FIG. 12 is a circuit diagramshowing a modification of the seventh preferred embodiment illustratedin FIG. 11.

FIG. 11 shows the tracking/focusing servo-error signal generatingcircuit in which at least three photo-detector units P1˜Pn (n denotesany natural number), e.g., photo-diode, in the above-mentioned sixthembodiment as shown in FIG. 10.

In a plurality of current-mirror circuits 8a˜8n(n denotes any naturalnumber), all the emitters of the reference transistors Q1A˜QnA areconnected in common to the power supply of the voltage E. Further, allthe emitters of the signal-output transistors Q1B˜QnB are connected incommon to the constant current source 5.

Further, the respective collectors of odd number of transistors Q1B,Q3B, . . . Q(2m+1)B (m also denotes any natural number) are connected tothe power supply of the voltage Vcc, via a resistor R1. Further, therespective collectors of even number of transistors Q2B, Q4B, . . . Q2mBare connected to the power supply of the voltage Vcc, via a resistor R2.

Further, similar to the case of FIG. 10, the output voltages from therespective nodes A7, B7 are amplified by a differential amplifier 12.Further, the respective collectors of the transistors Q1A˜QnA areconnected to the corresponding anodes of the photo-detectors P1˜Pn.

In this case, with respect to the currents flowing through therespective transistors Q1A˜QnA and Q1B˜QnB, the following equation (25)is obtained.

    IQ1A: IQ2A: IQ3A: . . . : IQnA =IQ1B: IQ2B: IQ3B : . . . : IQnB (25)

    IQ1B+IQ2B+IQ3B+. . . +IQnB=I                               (26)

Here, in connecting the outputs of the signal-output transistors Q1B˜QnBof the current-miller circuits 8a˜8n to the nodes A7, B7, variousconnection methods can be selected, under the condition that the numberof collectors connected to one node A7 must be equal to the number ofcollectors connected to the other node B7.

For example, the connection method of the collectors of thesignal-output transistors Q1B˜QnB when the number n is four (n=4) willbe considered. The related circuit (n=4) is illustrated in FIG. 11. Ifthe transistors Q1B˜Q4B are connected as shown in FIG. 11, the outputvoltage Vout is obtained as expressed in the following equation (27), ina similar manner to the equation (24). ##EQU4##

As apparent from the equation (27), it becomes possible for n (n=4)number of signals to be simultaneously controlled. Consequently, thecircuit configuration is simplified, and the size of the circuit can bereduced.

In such a construction, even though the respective absolute values ofthe input signals are different from each other, the collector currents,respectively corresponding to the ratios of the detection currentsIQ1A˜IQnA, can be accurately taken out. Therefore, it is possible tokeep a sum of the output currents from the current-mirror circuits 8a˜8ndefinite.

Further, by connecting the outputs of the signal-output transistorsQ1B˜QnB to the nodes A7, B7 under the condition that the number ofcollectors connected to one node A7 is equal to the number of collectorsconnected to the other node B7, it becomes possible for the desiredservo-error signals to be generated between the nodes A7, B7.

In FIG. 12, one of the current-mirror circuit blocks, in which theadditional (third) transistors QnC are respectively connected inparallel to the transistors QnB, is illustrated representatively.

As shown in FIG. 12, the respective emitters of the third transistorsQnC are coupled together and connected to a constant current source 5b(constant current I2 is generated). Further, each of the collectors ofthe third transistors QnC is adapted to be connected to the node whichthe collector of the transistor QnB on the same current-mirror circuitblock is not connected.

When such a connection is executed, the output signal expressed by thefollowing equation (28) is also obtained independently. ##EQU5##

As apparent from the equations (27), (28), the circuit shown in FIG. 12allows the different servo-error signals to be simultaneously generatedfrom the same photo-detector (e.g., P1). Further, even though therelation represented by I1+I2=I is not satisfied, two differentservo-error signals can be taken out.

Further, in the case where the circuit of FIG. 12 is combined with thecircuit of FIG. 13, it becomes possible for each of the collectors ofthe third transistors QnC is connected to the node which the collectorof the transistor QnB on the same circuit block is not connected.

Therefore, two kinds of servo-error signals, e.g., a trackingservo-error signal and a focusing servo-error signal, can besimultaneously obtained. Further, it becomes possible for the commonreference transistor of a single transistor to be provided, the areawhich will be occupied by a plurality of reference transistors in theprior art can be reduced.

FIG. 13 is a circuit diagram showing a eighth preferred embodimentaccording to the present invention. In this case, the circuits of FIG.12 are arranged in parallel by utilizing the transistors QnB, QnC andthe constant current sources 5a, ' 5b'.

In FIG. 13, all the emitters of the transistors Q1B ˜QnB are connectedin common to the constant current source 5a ' (the constant current I11is generated). Further, all the emitters of the transistors Q1C˜QnC areconnected in common to the constant current source 5b' (the constantcurrent I12 is generated). Further, the circuit is controlled so thatthe value of (I11+I12) is always constant (I1).

Further, the respective collectors of the transistors Q1B, Q2B, Q3C,Q4C, Q6B and Q6C are connected in common to a resistor R1, while therespective collectors of the transistors Q1C, Q2C, Q3B, Q4B, Q5B and Q5Care connected in common to a resistor R2. Further, the differentialvoltage between the resistors R1, R2 is amplified by the differentialamplifier 12, and finally the output voltage Vout as expressed by thefollowing equation (29) is obtained. ##EQU6## Where,

    α1=I11/(I11+I12)

    α2=I12/(I11+I12)

    α1+α2=1

Taking into account the fact that α1+α2=1, the equation (29) istransformed, and the following equation (30) is obtained. ##EQU7##

Further, it is assumed that α1-α2=(I11-I12)/(I11+I12)=α, and theequation (30) is transformed, and the following equation (31) isobtained. ##EQU8##

As apparent from the equation (31), by adequately changing the value ofα, the eighth embodiment has the advantage that the contribution factorsof the respective currents IQ1A, IQ2A, IQ3A and IQ4A can be easilyselected. In other words, in the case where the content of theservo-error signal is desired to be changed, it becomes possible foreach of the input signals to be given an adequate weight. The eighthembodiment shown in FIG. 13 corresponds to the construction in whicheach of the transistors QnB is divided into two parts and a sum of thecurrents I11, I12 is adjusted to a constant value in the eighthembodiment of FIG. 11. The eighth embodiment can be also utilized forthe servo control by six-divisional photo-detector.

FIG. 14 is a circuit diagram showing a ninth preferred embodimentaccording to the present invention; and FIG. 15 is a circuit diagramshowing a modification of the ninth preferred embodiment illustrated inFIG. 14.

In this case, the circuits of FIG. 14 are arranged by adding a pluralityof transistors QnD, QnE, constant current sources 5c, 5d, resistors R3,R4 and an operational amplifier 12' to the eighth embodiment of FIG. 13.

Here, the output voltage Vout' as expressed by the following equation(32) is obtained. ##EQU9##

Where, it is assumed that β=(I13-I14)/(I13+I14).

As apparent from the equation (32), the ninth embodiment can alsoutilized for the servo control by six-divisional photo-detector. To bemore specific, in the case where each content of two kinds ofservo-error signals is desired to be changed independently, it becomespossible for each of the input signals to be given an adequate weight sothat two kinds of servo-error signals do not interfere with each other.

Further, in FIG. 14, the base voltage are utilized in common in eachcurrent-mirror circuit block, and then the base current in thetransistors QnA˜QnE in each block is subtracted from the originaldetection current in each photo-detector. Therefore, the focusingservo-error signal obtained from the equation (31) and the trackingservo-error signal obtained from the equation (32) cause some numericalerrors.

To minimize such numerical errors, in FIG. 15, each of the transistorsQnx is connected in Darlington connection, and each of the base currentsin the transistors QnA˜QnE is supplied from the power supply of thevoltage Vcc, via each transistor Qnx.

Further, to execute turn-off operations of the transistors QnA˜QnE athigh speed, the by-pass current is adapted to flow from each base-lineto the emitter in each transistor QnA. In general, the resistor isconnected to the emitter of each of the transistors. However, in FIG.15, the by-pass circuit is formed by each transistor QnY. In such amethod, each transistor Qnx is prevented from having an influence on thedetection current as the error component, even in the case where thedetection current is extremely low. Namely, in the ninth embodiment,by-pass current can be adjusted in accordance with the level of thedetection current.

As described above, the present invention has been illustrated withrespect to several preferred embodiments applied to a magneto-opticaldisk device. However, the present invention is also applicable to anoptical disk device and an optical card, or the like, which utilizes anoptical beam for recording/reproducing operations.

While the present invention has been described as related to thepreferred embodiments, it will be understood that various changes andmodifications may be made without departing from the spirit and thescope of the invention as hereinafter claimed.

We claim:
 1. A positioning control system utilizing an optical beam inwhich recording/reproducing operations are performed by irradiating saidoptical beam to a predetermined position on an optically recordingmedium, comprising:a photo-detector constituted by at leasttwo-divisional photo-detector units; and a servo-error signal generatingcircuit which generates at least one servo-error signal for servocontrol of said optical beam in accordance with a difference betweenrespective detection currents detected by said photo-detector units fordetecting return optical beam reflected from said recording medium, soas to accurately irradiate an original optical beam to desired positionon the basis of said servo-error signal, wherein said servo-error signalgenerating circuit includes: at least one division circuit that has twopairs of transistors, two emitters in each pair of said two pairs oftransistors being connected together into a common emitter, biasvoltages of direct current type being applied to the respective basescorresponding to transistors on one side in the respective pairs oftransistors, the respective collectors corresponding to said transistorson one side in the respective pairs of transistors being connected to acommon connecting portion via corresponding resistors, said commonconnecting portion being connected to a power supply, the respectivebases corresponding to transistors on the other side in the respectivepairs of transistors being connected together into a common base; and anintegrating capacitor which is connected to said common basecorresponding to said transistors on the other side, and whichintegrates a difference between the current equivalent to the currentflowing through said common connecting portion of said respectivecollectors and the predetermined reference current, to apply the thusintegrated different to said common base corresponding to saidtransistors on the other side, wherein said photo-detector units arerespectively connected to said common emitters in the respective pairsof said transistors, so that said respective detection currents can flowthrough said common emitters, and wherein said division circuit isoperative to obtain said servo-error signal from the respectivepotentials of said respective collectors corresponding to saidtransistors on one side.
 2. A positioning control system as set forth inclaim 1, wherein said common connecting portion is connected to saidpower supply via a current-mirror circuit, and wherein said integratingcapacitor is operative to integrate a difference between the mirrorcurrent generated in said current-mirror circuit and said referencecurrent, and to apply the thus integrated difference to said common basecorresponding to said transistors on the other side.
 3. A positioningcontrol system as set forth in claim 1, wherein said servo-error signalgenerating circuit is constituted by an integrated circuit.
 4. Apositioning control system as set forth in claim 1, wherein it furthercomprises at least one operational amplifier which calculates said adifference between the respective potentials of said respectivecollectors corresponding to said transistors on one side.
 5. Apositioning control system as set forth in claim 1, wherein it isapplied to a magneto-optical disk device which comprises:amagneto-optical disk which functions as said optically recording mediumand which is rotated by a motor; an optical head which irradiates saidoptical beam to predetermined position on said magneto-optical disk forrecording/reproducing operations, and which is adapted to be driven byanother motor so as to move with respect to radial direction of saidmagneto-optical disk.
 6. A positioning control system as set forth inclaim 1, wherein it is applied to an optical disk device whichcomprises:an optical disk which functions as said optically recordingmedium and which is rotated by a motor; an optical head which irradiatessaid optical beam to predetermined position on said optical disk forrecording/reproducing operations, and which is adapted to be driven byanother motor so as to move with respect to radial direction of saidoptical disk.